The present invention relates to a holographic planar optic display system and, in particular, to a multi chromatic holographic planar optic display system, which employs a planar optic approach, and which include a Shift Adjusted Display (SAD) device.
As used herein the specifications and claims, the term Shift Adjusted Display (SAD) device refers to a device that performs electronic diversion of input image pixels to the Chromatic Planar Optic Display System, for the purpose of improving the output display image as a result of distorted optic transformation, which is unwanted in the optic display system.
There are a wide variety of display systems for visual radiation of a video image to the user's eyes. The video image can reach the display system from a wide range of sources, such as video cameras, DVD, VCR, computers, and receiver antennas, as a video signal in wire or wireless communication.
These video signals are command signals for the recreation of the image display using a light-radiating device. Light-radiating display devices are based on technologies such as: Cathode Ray Tube (CRT), Light Emitting Diode (LED), Liquid Crystal Display (LCD), Passive Matrix LCD (PMLCD), Active Matrix LCD (AMLCD), Active Matrix Electro-Luminescent display (AMEL), Micro-Electro Mechanical display (MEM), Thin Film Transistors (TFT), Light Doped Drain (LDD), Liquid Crystal On Silicon (LCOS), Ferroelectric LCD (FLC).
Examples of application of these devices are television sets, computer screens, billboards, wristwatches, handheld computers, cellular phones, etc. However, there still are certain situations in which looking directly at a display screen is insufficient. Such situations include Head Up Display (HUD) and Visor Display, in aircraft and other vehicles in which the outer world is seen through image display, or in Head Mounted Display (HMD), in which the system is small to the extent that the eye is unable to focus on the image, on account of the distance between the eye and the screen being too short.
A displayed image may be either a real image or a virtual image. A real image refers to an image, which is observed directly by the unaided human eye, displayed by a viewing surface positioning at a given place. Compact display devices, due to their small size, have a limited surface area on which a real image can be provided. Since the amount of detail that the human eye can resolve per unit of area is limited, devices, which provide a real image, are only capable to providing a limited amount of legible information per display screen.
Due to current technological trends and developments, there is a growing demand for a mobile and compact display device that is capable of displaying an increasing number of visual data to the viewer, which is expressed in a growing demand for a display device with a small surface area and an increasing number of display pixels.
One approach to reduce the size of an image display and yet retain image quality is through the formation of a virtual image instead of a real image. A virtual image can exist at a place where no display surface exists.
A virtual image is seen through an optical device. An example of a virtual image is the image viewed through a magnifying glass, a three-dimensional hologram, a mirror-reflected image, the image viewed through a combiner of head up display and the image viewed through an output diffractive optical element of planar optic visor display. Virtual image displays can provide an image, which appears to be larger than the source object from which the virtual image emerges, and at a distance suitable for the focus ability of the eye.
Creation of the image in Compact Display Sources can be performed in one of several methods, such as scanning a screen with an electron ray in CRT monitors in parallel lines at a high speed, or radiation of light from Chip Laser Emitted Diodes (LED) serving as backlights in flat screen Liquid Crystal Displays (LCD). The result is an image displayed to the viewer's eyes, which is comprised of many light-radiating pixels, aligned in most devices as a two dimensional array of pixels. A light-radiating pixel of this type is also known as a dot.
A dot or pixel is an element that forms a character or symbol when combined in a matrix, or an array. In monochromatic displays, every such pixel can either project light or not, and in more advanced systems the light radiation can be at several grades of intensity, which allows for pixel gray level.
Color display systems have the option of creating a pixel referred to as a full-color pixel, as a combination of several light-radiating pixels, which are in close proximity to each other, while each radiates light in an additive primary color, at the necessary intensity. The combination of these additive primary colors at the necessary division of intensities gives the feel of a color pixel of the desired hue and brightness. The term “additive” refers to the addition of several primary colors, usually three: red, blue, and green. at the appropriate ratios to create the sense of a color of any hue and brightness within the color vision spectrum. Color addition is suitable for creation of a color image from light-radiating sources
Another method of creating color images is “subtraction”. In this method, the source of light can be white sunlight, which is blocked with three filters at the necessary filtering intensity, usually yellow, red, and blue filters. An example of use of the subtraction method is in watercolor paintings.
Another option is radiating light in primary colors in the sequential color method. In this method, each pixel in the display array radiates the additive primary colors (red, green, and blue) at a high rate that gives the eye the sense of simultaneous radiation, and the end result of a color pixel of the desired hue and brightness.
An example of a Compact Display Source on the market nowadays is the AMLCD CyberDisplay 640 Color, manufactured by the Kopin Corporation, 695 Myles Standish Blvd., Taunton, Mass. USA. Its active display area measures 5.76 mm×7.68 mm with VGA resolution 640×480 pixels and a video rate of up to 180 frames per second, with three primary colors: red, green, and blue, and a filed of view of 32 degrees. This compact display source is based on Kopin's field color sequential technology in which time division multiplexing produces color by rapidly creating a repetitive sequence of red, green and blue sub-images which the human eye integrates into a full color image.
There are several standards of display arrays, in which one of the primary characteristics is the increase in number of pixels, for example:
Color Graphics ArrayCGA200 × 640 pixelsEnhanced Graphics ArrayEGA350 × 640 pixelsVideo Graphics ArrayVGA480 × 640 pixelsSuper Video Graphics ArraySVGA600 × 800 pixels
As noted above, the present invention relates to holographic planar optic display system. The holographic planar optic display device is a highly efficient display device, as it is both compact and inexpensive. The general structure of the holographic planar optic display device is described in FIG. 1.
This basic structure uses two Diffractive Optical Elements (DOE). Structures of holographic planar optic display devices can include a higher number of DOE's. In display systems based on geometrical optics, the diversion of light rays is made by use of lenses, beam splitters, and mirrors, which cause a relatively thick display system. In planar optic display systems on the other hand, which are based on diffractive optics, the DOE serves as an optical grating that diverts the light coming through it (or reflected from it) by taking advantage of the diffraction phenomenon.
The light is diverted by the input element at a sufficiently large angle to enable it, upon hitting the transparent substrate plate, to be reflected in full internal reflection and move in the substrate until it reaches the output element, which diverts it out of the substrate. The gratings can be light-transmissive gratings or light-reflective gratings. The thickness of the grating is negligible in comparison to those of geometrical optical elements, and the transparent substrate plate thickness is small, approximately 1–3 mm. therefore, display systems based on diffractive planar optics can be extremely compact.
These systems are particularly good when the display is in monochromatic light, namely light with only one wavelength or frequency. In this case, use of simple linear gratings is sufficient, as described in U.S. Pat. No. 4,711,512 of Upatnieks, the contents of which are hereby incorporated by reference.
The light is radiated from the image source in several angles; each pixel radiates a beam of light, as a cone with a certain angle of opening. In planar optic systems, the light radiated from the image source usually undergoes collimation by a geometrical lens placed between the image source and the input element or within the input element. Another possibility is partial collimation in the geometrical lens or the input element and additional collimation by additional gratings between the input element and the output element. After collimation the rays of light are parallel to each other.
The light diversion angle in the grating depends on the structure of the grating, the ratio between the index of refraction of the transparent substrate and the index of refraction of the environment and the light's wavelength. A simple linear grating diverts each wavelength at a different angle. Therefore a system with a linear input grating will create lateral chromatic aberration at its output.
When planning the grating spacing of the input grating, in use of linear gratings, the light must pass through it or be reflected from it at the appropriate angle, which is called the critical angle βC, which assures that the light will be reflected from the inner side of the transparent substrate plate in total internal reflection.
The minimal angle, which is called the critical angle βC that assures total internal reflection of the light within the substrate, can be calculates using Snell's law, that states that for a system in air, βC=sin−1(1/np), np being the plate index of refraction. For example, in glass this index is approximately 1.51.
In color display systems this calculation will be made for the light with the shortest wavelength. For blue light with a wavelength of λB, the angle βB will be calculated, as it is diverted at the smallest angle. The desired grating spacing (gs) can be calculated using the equation: np sin βB−na sin βi=λB/gs or when βi=0, when light hits the input grating at a perpendicular angle: np sin βB=λB/gs
Now, the diversion angles of the other light beams can be calculated, and we can find, for example, the diversion angle for green light, βG, and the diversion angle for red light, βR, and the cycle distance (cd) of each of the light's color  components. Obviously, the light rays will each arrive to the output grating at a different distance from the substrate input point and after a different number of cycles.
Over the past few years, many efforts to solve the problem of chromatic aberration in planar optic display systems. The solutions offered also include the use of more than two gratings or use of a complex input grating. Examples of this are described in U.S. Pat. No. 5,966,223 of Friesem et al, which describes the use of a complex input grating, as illustrated in FIG. 4a; in PCT International Publication No. WO 99/52002 to Amitai et al, which describes the use of at least three gratings, and the second grating diverts the light on the substrate by 90° as illustrated in FIG. 4b; in PCT International Publication No. WO 01/09663 to Friesem et al, which describes the addition of at least one additional diffractive optical element being positioned between the input and the output diffractive optical elements, as illustrated in FIG. 4c; and in PCT International Publication No. WO 01/95027 to Amitai, which describes the installment of a reflecting surface in the input and the installment of a parallel array of partially reflecting surfaces, as illustrated in FIG. 4d, which necessitates thickening the transparent substrate. The contents of these four examples are hereby incorporated by reference.
These solutions are limited in their ability to solve the problem of chromatic dispersion, and mostly the problem of chromatic aberration, and also complicate the production process of the display systems.
There is therefore a need for, and it would be highly advantageous to have a compact multichromatic display system in which the image is displayed to the viewer without unwanted chromatic aberration and/or chromatic dispersion.